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Investments - Additional Information (Detail)
12 Months Ended
Dec. 31, 2019
USD ($)
Integer
Dec. 31, 2018
USD ($)
Integer
Dec. 31, 2017
USD ($)
Integer
Sep. 30, 2018
USD ($)
Schedule of Investments        
Fixed Maturity Securities Other Than Temporary Impairments in Accumulated Other Comprehensive Income Loss $ 0 $ 0    
Commitment to Fund Private Placement Fixed Maturity Securities 98,700,000      
Carrying Amount of Variable Interest Entity Investments 675,100,000 575,300,000    
Unfunded Commitments 1,900,000      
Fixed Maturity Securities 47,443,700,000 43,011,700,000    
Other Long-term Investments $ 844,200,000 702,900,000    
Mortgage Loan Policy is not to Exceed a Loan-to-value Ratio, Percent 75.00%      
Loan to Value Ratio Update Frequency three      
Mortgage Loan Inspection Frequency two      
Mortgage Loan Policy Debt Service Coverage Ratio, Lower Range 125.00%      
Mortgage Loans Issuance, Term Years 25      
Number of Changes to Accounting Policy for Estimating Credit Losses on Mortgage Loans 0      
Financing Receivable, Allowance for Credit Losses     $ 0  
Foreclosed Real Estate Expense $ 0 0 0  
Average Recorded Investment of Impaired Mortgage Loans 600,000 2,300,000    
Interest Income on Mortgage Loans Subsequent to Impairment 0 0 0  
Committments to Fund Commercial Mortgage Loans $ 56,000,000.0      
Repurchase agreements - Typical Days outstanding 30      
Minimum percent of the fair value of securities loaned or securities purchased under repurchase agreements be maintained as collateral 102.00%      
Gross Securities Borrowed $ 176,400,000 164,100,000    
Outstanding Repurchase Agreements 0 0    
Off Balance Sheet Amount        
Schedule of Investments        
Cash Collateral for Borrowed Securities   100,000    
Securities Received as Collateral 186,500,000 171,400,000    
Commercial Real Estate Portfolio Segment        
Schedule of Investments        
Financing Receivable, Allowance for Credit Losses       $ 200,000
Loans and Leases Receivable, Allowance 200,000 200,000 0  
Provision for Loan and Lease Losses 0 200,000 $ 0  
Financing Receivable, Recorded Investment, 90 Days Past Due and Still Accruing 0 0    
Commercial Real Estate [Member]        
Schedule of Investments        
Provision for Loan and Lease Losses   100,000    
Fixed Maturity Securities        
Schedule of Investments        
Fixed Maturity Securities $ 47,443,700,000 $ 43,011,700,000    
Commercial Real Estate Portfolio Segment        
Schedule of Investments        
Financing Receivable, Modifications, Subsequent Default, Number of Contracts | Integer     0  
Financing Receivable, Modifications, Number of Contracts | Integer 0 1 0  
Financing Receivable, Modifications, Pre-Modification Recorded Investment   $ 3,600,000    
Special Purpose Entity | Bonds        
Schedule of Investments        
Fixed Maturity Securities   156,700,000    
Partnership Interests In Tax Credit Investments        
Schedule of Investments        
Carrying Amount of Variable Interest Entity Investments $ 58,400,000 91,500,000    
Equity Method Investments        
Schedule of Investments        
Carrying Amount of Variable Interest Entity Investments $ 616,700,000 $ 483,800,000    
External Credit Rating, Investment Grade        
Schedule of Investments        
Number of Fixed Maturity Securities in an Unrealized Loss Position 109      
Number of Fixed Maturity Securities in an Unrealized Loss Position Continuously for Over One Year 19      
Fixed Maturity Securities $ 44,255,200,000      
External Credit Rating, Below-Investment-Grade        
Schedule of Investments        
Number of Fixed Maturity Securities in an Unrealized Loss Position 38      
Number of Fixed Maturity Securities in an Unrealized Loss Position Continuously for Over One Year 24      
Fixed Maturity Securities $ 3,188,500,000